The invention relates to a process for melting inorganic materials, in particular ashes, slags, glass and metals in a rotary horizontal converter or a rotary kiln.
In metallurgy, various processes are known for melting solid materials. The energy which is required for melting the solids is provided here either via a fossil-fueled heater or an electrical heater.
Fossil-fueled heaters generally use burners which are charged with liquid, gaseous or pulverized fuels. To oxidize the fuel, in addition to air, oxygen and oxygen-enriched air are also used. The latter is advantageous if materials having a high melting temperature, for example metallurgical slags, are to be melted. The flame temperature which can be achieved is considerably higher in this case than when air is used. In addition, the amount of exhaust gas is less, which leads to lower losses of exhaust gas and to fuel savings.
The disadvantage of the fossil-fueled burner heaters is that the heat generated can only be transmitted via the surface of the material to be melted. The heat-transfer surface is limited by this. If the melting process is carried out in non-agitated converters, in addition, the heat conduction in the bath and the internal bath convection are small.
These findings have led to the development of what are termed underbath burners. In these, a cooled burner lance is immersed directly in the bath, and the fuels are injected into the bath together with an oxidizing agent. A disadvantage in this case is the complex cooling, which is not without hazard, of these burners and the limited depth of penetration which can be achieved. Only gaseous and liquid fuels can be used.
In contrast, in the case of the electric melting processes (resistance, induction, arc and plasma heaters), high temperatures can advantageously be reached, and the exhaust gas losses are eliminated. However, a disadvantage is the high energy and plant costs.
Accordingly, one object of the invention is to provide a novel process for melting inorganic materials, which avoids these disadvantages, permits the use of inexpensive fuels and by which, nevertheless, the required high temperatures are achieved.
According to the invention, this is achieved by a process for melting inorganic materials in a rotary horizontal converter or a rotary kiln by adding fuel, oxygen or oxygen-enriched gases and the inorganic materials to a liquid bath. The solid fuel is added in piece form and the oxygen or the oxygen-enriched gas is blown at high velocity onto the bath surface, said gas reacting with the solid fuel and releasing heat during the process.
The advantages of the invention are that, for melting the inorganic materials, inexpensive fuels can be used.
In addition, owing to the oxygen impingement of the bath at high velocity, an intensive stirring action is generated at the bath surface. The use of a rotary converter or a rotary tubular kiln ensures that the material to be heated is continuously agitated and mixed. Unmolten material floats in this case, as a result of lower density, preferably at the surface and there comes intensively into contact with the hot oxidizing fuels and combustion gases.
It is particularly expedient if the velocity at which the oxygen or the oxygen-enriched gas is blown onto the bath surface is at least as high as the velocity of sound. Then, intensive stirring action and, resulting therefrom, good mixing of the reacting materials is ensured.
It is advantageous if the fuel used is coal or carboniferous fuel and the coal or the carboniferous fuel and the oxygen are fed continuously, since by this means the redox state in the gas phase and also the redox state in the bath can be set as desired via the ratio of coal to oxygen. In particular, reducing conditions can be set with stoichiometric ratios of carbon to oxygen of greater than 1 in order to reduce heavy metals in the bath to their metallic form and evaporate them off.
It is further expedient if the fuel used is dry or predried wastes. This is a particularly cheap variant.
It is advantageous if the furnace is operated in such a manner that, during startup, a bath of molten material is charged, which molten material is either generated by using conventional burners which are operated using liquid, gaseous or pulverized fuels, or is introduced by charging from a further melting furnace. Not until thereafter are piece-type fuel and the material to be melted fed to the converter and impinged with oxygen, the oxygen reacting with the solid fuel and heat being released which leads to melting of the material used. Owing to the fact that during startup of the furnace a liquid bath of molten material is already present, firstly the piece-type fuel, from the start, is situated at the surface of the melt and, secondly, no further precautions with regard to ignition of the piece-type fuel subsequently used need to be taken.
If metals are to be melted by the process of the invention, it is advantageous if the converter is charged with two phases, a metal phase and a slag phase thereabove, the injected oxygen being brought to reaction with the fuel on or in the slag phase. This prevents the injected oxygen coming into contact directly with the molten or still unmolten metal. The oxygen only reacts with the fuel on the slag phase. This releases the heat which is necessary for melting the still solid metal.
Furthermore, it is advantageous if grate ash from refuse incineration is fused by direct combustion of dry, predried, pyrolyzed or gasified wastes or residues, where, together with the grate ash, boiler ash and/or filter ash from refuse incineration can also be fused.